Multiple sclerosis (MS) is a chronic demyelinating disease of unknown cause. Attempts to link the retrovirus human T-cell lymphotropic virus I (HTLV-I) to MS have been made by the demonstration of DNA sequences in peripheral blood leukocytes of MS patients that show sequence homology to the HTLV-I genome. We have continued to investigate the finding of such sequences in post mortem brain tissue and control subjects.